Combined cycle power plants generally comprise a gas turbine, a steam turbine, one or more generators which are driven by the turbines and a heat recovery steam generator (HRSG) for generating steam for the steam turbine using the gas turbine exhaust gasses. In the past, such combined cycle power plants have been controlled by varying the fuel flow to the gas turbine in response to variations in the power plant load. In such an arrangement, an increase in fuel flow to the gas turbine increases gas turbine power output and increases the flow exhaust gasses from the gas turbine to the HRSG and the temperature of such gasses. This, in turn, increases the temperature of the steam generated by the HRSG and, hence, the power output of the steam turbine. Such prior arrangement has been highly satisfactory in applications that are not subject to wide swings in power plant loading. For example, such prior arrangement has been successfully applied in electrical utility applications. However, in applications where the load is subject to wide and rapid variations, as for example, where the power plant generates electricity for a steel mill having rolling mills and/or arc furnaces, such prior arrangement would result in temperature variations in the gas turbine, HRSG and steam turbine, with the possibility of creating high thermal stresses and reduced life.
A primary object of the present invention is to provide a combined cycle power plant arrangement that is tolerant of rapidly fluctuating loads.
Another object of this invention is a combined cycle power plant arrangement that can accommodate wide variations in power plant loading without producing wide temperature swings in the gas turbine. HRSG and steam turbine.